Design of New Ligands for the Palladium-Catalyzed Arylation of α-Branched Secondary Amines

A General Strategy for N-(Hetero)arylpiperidine Synthesis Using Zincke Imine Intermediates

Methods to synthesize diverse collections of substituted piperidines are valuable due to the prevalence of this heterocycle in pharmaceutical compounds. Here, we present a general strategy to access N-(hetero)arylpiperidines using a pyridine ring-opening and ring-closing approach via Zincke imine intermediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydrogenation reactions and nucleophilic additions then access the N-(hetero)arylpiperidine derivatives. We successfully applied high-throughput experimentation (HTE) using pharmaceutically relevant pyridines and (heteroaryl)anilines as inputs and developed a one-pot process using anilines as nucleophiles in the pyridinium salt-forming processes. This strategy is viable for generating piperidine libraries and applications such as the convergent coupling of complex fragments.

A machine-learning tool to predict substrate-adaptive conditions for Pd-catalyzed C-N couplings

Machine-learning methods have great potential to accelerate the identification of reaction conditions for chemical transformations. A tool that gives substrate-adaptive conditions for palladium (Pd)-catalyzed carbon-nitrogen (C-N) couplings is presented. The design and construction of this tool required the generation of an experimental dataset that explores a diverse network of reactant pairings across a set of reaction conditions. A large scope of C-N couplings was actively learned by neural network models by using a systematic process to design experiments. The models showed good performance in experimental validation: Ten products were isolated in more than 85% yield from a range of couplings with out-of-sample reactants designed to challenge the models. Importantly, the developed workflow continually improves the prediction capability of the tool as the corpus of data grows.

Important Role of NH-Carbazole in Aryl Amination Reactions Catalyzed by 2-Aminobiphenyl Palladacycles

2-Aminobiphenyl palladacycles are among the most successful precatalysts for Pd-catalyzed cross-coupling reactions, including aryl amination. However, the role of NH-carbazole, a byproduct of precatalyst activation, remains poorly understood. Herein, the mechanism of the aryl amination reactions catalyzed by a cationic 2-aminobiphenyl palladacycle supported by a terphenyl phosphine ligand, PCyp₂Ar^Xyl2 (Cyp = cyclopentyl; Ar^Xyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl), P1, has been thoroughly investigated. Combining computational and experimental studies, we found that the Pd(II) oxidative addition intermediate reacts with NH-carbazole in the presence of the base (NaO ^t Bu) to yield a stable aryl carbazolyl Pd(II) complex. This species functions as the catalyst resting state, providing the amount of monoligated LPd(0) species required for catalysis and minimizing Pd decomposition. In the case of a reaction with aniline, an equilibrium between the carbazolyl complex and the on-cycle anilido analogue is established, which allows for a fast reaction at room temperature. In contrast, heating is required in a reaction with alkylamines, whose deprotonation involves coordination to the Pd center. A microkinetic model was built combining computational and experimental data to validate the mechanistic proposals. In conclusion, our study shows that despite the rate reduction observed in some reactions by the formation of the aryl carbazolyl Pd(II) complex, this species reduces catalyst decomposition and could be considered an alternative precatalyst in cross-coupling reactions.

Pd-Catalyzed Amination of Base-Sensitive Five-Membered Heteroaryl Halides with Aliphatic Amines

We report a versatile and functional-group-tolerant method for the Pd-catalyzed C-N cross-coupling of five-membered heteroaryl halides with primary and secondary amines, an important but underexplored transformation. Coupling reactions of challenging, pharmaceutically relevant heteroarenes, such as 2-H-1,3-azoles, are reported in good-to-excellent yields. High-yielding coupling reactions of a wide set of five-membered heteroaryl halides with sterically demanding α-branched cyclic amines and acyclic secondary amines are reported for the first time. The key to the broad applicability of this method is the synergistic combination of (1) the moderate-strength base NaOTMS, which limits base-mediated decomposition of sensitive five-membered heteroarenes that ultimately leads to catalyst deactivation, and (2) the use of a GPhos-supported Pd catalyst, which effectively resists heteroarene-induced catalyst deactivation while promoting efficient coupling, even for challenging and sterically demanding amines. Cross-coupling reactions between a wide variety of five-membered heteroaryl halides and amines are demonstrated, including eight examples involving densely functionalized medicinal chemistry building blocks.

Buchwald-Hartwig Amination of Coordinating Heterocycles Enabled by Large-but-Flexible Pd-BIAN-NHC Catalysts*

A new class of large-but-flexible Pd-BIAN-NHC catalysts (BIAN=acenaphthoimidazolylidene, NHC=N-heterocyclic carbene) has been rationally designed to enable the challenging Buchwald-Hartwig amination of coordinating heterocycles. This robust class of BIAN-NHC catalysts permits cross-coupling under practical aerobic conditions of a variety of heterocycles with aryl, alkyl, and heteroarylamines, including historically challenging oxazoles and thiazoles as well as electron-deficient heterocycles containing multiple heteroatoms with BIAN-INon (N,N'-bis(2,6-di(4-heptyl)phenyl)-7H-acenaphtho[1,2-d]imidazol-8-ylidene) as the most effective ligand. Studies on the ligand structure and electronic properties of the carbene center are reported. The study should facilitate the discovery of even more active catalyst systems based on the unique BIAN-NHC scaffold.

Synthesis of Azidoanilines by the Buchwald-Hartwig Amination

We report a Buchwald-Hartwig amination compatible with azido functionality. Treatment of azidoaryl iodides and amines with fourth-generation Buchwald precatalyst coordinated by CPhos and sodium tert-butoxide in 1,4-dioxane at 50 °C afforded the corresponding azidoanilines while leaving the azido groups intact. The method showed a broad substrate scope and was applicable to the synthesis of diazido compounds as photoaffinity probe candidates of pharmaceutical amines and multiazido platform molecules.

Chiral Arylated Amines via C-N Coupling of Chiral Amines with Aryl Bromides Promoted by Light

The Buchwald-Hartwig C-N coupling reaction has found widespread applications in organic synthesis. Over the past two decades or so, many improved catalysts have been introduced, allowing various amines and aryl electrophiles to be readily used nowadays. However, there lacks a protocol that could be used to couple a wide range of chiral amines and aryl halides, without erosion of the enantiomeric excess (ee). Reported in this article is a method based on molecular Ni catalysis driven by light, which enables stereoretentive C-N coupling of optically active amines, amino alcohols, and amino acid esters with aryl bromides, with no need for any external photosensitizer. The method is effective for a wide variety of coupling partners, including those bearing functional groups sensitive to bases and nucleophiles, thus providing a viable alternative to accessing synthetically important chiral N-aryl amines, amino alcohols, and amino acids esters. Its viability is demonstrated by 92 examples with up to 99 % ee.

Combined Experimental and Computational Mechanistic Investigation of the Palladium-Catalyzed Decarboxylative Cross-Coupling of Sodium Benzoates with Chloroarenes

Reported herein is a mechanistic investigation into the palladium-catalyzed decarboxylative cross-coupling of sodium benzoates and chloroarenes. The reaction was found to be first-order in Pd. A minimal substituent effect was observed with respect to chloroarene, and the reaction was zero-order with respect to chloroarene. Palladium-mediated decarboxylation was assigned as the turnover-limiting step based on an Eyring plot and density functional theory computations. Catalyst performance was found to vary based on the electrophile, which is best explained by catalyst decomposition at Pd(0). The 1,5-cyclooctadiene (COD) ligand contained in the precatalyst CODPd(CH₂TMS)₂ (Pd1) was shown to be a beneficial additive. The bench-stable Buchwald complex XPhosPdG2 could be used with exogenous COD and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (XPhos) instead of complex Pd1. Adding exogenous XPhos significantly increased the catalyst turnover number and enhanced reproducibility.

(DiMeIHeptCl)Pd: A Low-Load Catalyst for Solvent-Free (Melt) Amination

(DiMeIHept^Cl)Pd, a hyper-branched N-aryl Pd NHC catalyst, has been shown to be efficient at performing amine arylation reactions in solvent-free ("melt") conditions. The highly lipophilic environment of the alkyl chains flanking the Pd center serves as lubricant to allow the complex to navigate through the paste-like environment of these mixtures. The protocol can be used on a multi-gram scale to make a variety of aniline derivatives, including substrates containing alcohol moieties.

Modular Synthesis of a Versatile Double-Allylation Reagent for Complex Diol Synthesis

Double-allylation reagents allow for the construction of highly complex molecules in an expedient fashion. We have developed an efficient, modular, and enantioselective approach towards accessing novel variants of these reagents through Cu/Pd-catalyzed alkenylboration of alkenylboron derivatives. Importantly, we demonstrate novel use of an allylBdan reagent directly in a stereocontrolled allylation without initial deprotection to the boronic ester. These allylation products are employed in a second intermolecular allylation to access complex diol motifs, which has yet to be shown with these types of double-allylation reagents. Overall, the modularity of this approach and the ease in which complex structural motifs can be accessed in a rapid manner signify the importance and utility of this method.

Low-Temperature Nickel-Catalyzed C-N Cross-Coupling via Kinetic Resolution Enabled by a Bulky and Flexible Chiral N-Heterocyclic Carbene Ligand

The transition-metal-catalyzed C-N cross-coupling has revolutionized the construction of amines. Despite the innovations of multiple generations of ligands to modulate the reactivity of the metal center, ligands for the low-temperature enantioselective amination of aryl halides remain a coveted target of catalyst engineering. Designs that promote one elementary reaction often create bottlenecks at other steps. We here report an unprecedented low-temperature (as low as -50 °C), enantioselective Ni-catalyzed C-N cross-coupling of aryl chlorides with sterically hindered secondary amines via a kinetic resolution process (s factor up to >300). A bulky yet flexible chiral N-heterocyclic carbene (NHC) ligand is leveraged to drive both oxidative addition and reductive elimination with low barriers and control the enantioselectivity. Computational studies indicate that the rotations of multiple σ-bonds on the C₂ -symmetric chiral ligand adapt to the changing needs of catalytic processes. We expect this design would be widely applicable to diverse transition states to achieve other challenging metal-catalyzed asymmetric cross-coupling reactions.

Carbamate and N-Pyrimidine Mitigate Amide Hydrolysis: Structure-Based Drug Design of Tetrahydroquinoline IDO1 Inhibitors

Indoleamine-2,3-dioxygenase-1 (IDO1) has emerged as an attractive target for cancer immunotherapy. An automated ligand identification system screen afforded the tetrahydroquinoline class of novel IDO1 inhibitors. Potency and pharmacokinetic (PK) were key issues with this class of compounds. Structure-based drug design and strategic incorporation of polarity enabled the rapid improvement on potency, solubility, and oxidative metabolic stability. Metabolite identification studies revealed that amide hydrolysis in the D-pocket was the key clearance mechanism for this class. Strategic survey of amide isosteres revealed that carbamates and N-pyrimidines, which maintained exquisite potencies, mitigated the amide hydrolysis issue and led to an improved rat PK profile. The lead compound 28 is a potent IDO1 inhibitor, with clean off-target profiles and the potential for quaque die dosing in humans.

Tungsten-Catalyzed Direct N-Alkylation of Anilines with Alcohols

The implementation of non-noble metals mediated chemistry is a major goal in homogeneous catalysis. Borrowing hydrogen/hydrogen autotransfer (BH/HA) reaction, as a straightforward and sustainable synthetic method, has attracted considerable attention in the development of non-noble metal catalysts. Herein, we report a tungsten-catalyzed N-alkylation reaction of anilines with primary alcohols via BH/HA. This phosphine-free W(phen)(CO)₄ (phen=1,10-phenthroline) system was demonstrated as a practical and easily accessible in-situ catalysis for a broad range of amines and alcohols (up to 49 examples, including 16 previously undisclosed products). Notably, this tungsten system can tolerate numerous functional groups, especially the challenging substrates with sterically hindered substituents, or heteroatoms. Mechanistic insights based on experimental and computational studies are also provided.

Synthesis, characterization, and coordination chemistry of a phenanthridine-containing N-heterocyclic carbene ligand

An N-heterocyclic carbene ligand precursor bearing a π-extended phenanthridine (3,4-benzoquinoline) unit is presented. The proligand was isolated as the imidazolium salt of chloride (1•HCl), bromide (1•HBr), or hexafluorophosphate (1•HPF6) counterions. These salts can be deprotonated and the carbene installed on silver centres using Ag2O as both a base and a source of metal ion. The resulting Ag(I) complex (1)AgCl can be used in a transmetalation reaction to generate a Pd(II) coordination complex (1)Pd(CH3CN)Cl2. The characterization and photophysical properties of these complexes is presented, along with a demonstration of the utility of (1)Pd(CH3CN)Cl2 in mediating a catalytic C-N cross-coupling reaction for the preparation of the pharmaceutical Piribedil.

A post-synthetically modified metal–organic framework for copper catalyzed denitrative C–N coupling of nitroarenes under heterogeneous conditions

Post-synthesis modification of DMOF, afforded a desired material for strategic infusion of catalytically active centers in a porous matrix. The catalyst is capable for denitrative C–N coupling reactions of nitroarenes under heterogeneous conditions.

Cu-Catalyzed C–N Coupling with Sterically Hindered Partners

Copper, an earth-abundant metal, has reemerged as a viable alternative to the versatile Pd-catalyzed C-N coupling. Coupling sterically hindered reaction partners, however, remains challenging. Herein, we disclose the discovery and development of a pyrrole-ol ligand to facilitate the coupling of ortho-substituted aryl iodides with sterically hindered amines. The ligand was discovered through a library screening approach and highlights the value of mining heteroatom-rich pharmaceutical libraries for useful ligand motifs. Further evaluation revealed that this ligand is uniquely effective in these challenging transformations. The reaction enables the coupling of sterically hindered primary and secondary amines, anilines, and amides with broad functional group tolerance.

Development of an Aryl Amination Catalyst with Broad Scope Guided by Consideration of Catalyst Stability

We have developed a new dialkylbiaryl monophosphine ligand, GPhos, that supports a palladium catalyst capable of promoting carbon-nitrogen cross-coupling reactions between a variety of primary amines and aryl halides; in many cases, these reactions can be carried out at room temperature. The reaction development was guided by the idea that the productivity of catalysts employing BrettPhos-like ligands is limited by their lack of stability at room temperature. Specifically, it was hypothesized that primary amine and N-heteroaromatic substrates can displace the phosphine ligand, leading to the formation of catalytically dormant palladium complexes that reactivate only upon heating. This notion was supported by the synthesis and kinetic study of a putative off-cycle Pd complex. Consideration of this off-cycle species, together with the identification of substrate classes that are not effectively coupled at room temperature using previous catalysts, led to the design of a new dialkylbiaryl monophosphine ligand. An Ot-Bu substituent was added ortho to the dialkylphosphino group of the ligand framework to improve the stability of the most active catalyst conformer. To offset the increased size of this substituent, we also removed the para i-Pr group of the non-phosphorus-containing ring, which allowed the catalyst to accommodate binding of even very large α-tertiary primary amine nucleophiles. In comparison to previous catalysts, the GPhos-supported catalyst exhibits better reactivity both under ambient conditions and at elevated temperatures. Its use allows for the coupling of a range of amine nucleophiles, including (1) unhindered, (2) five-membered-ring N-heterocycle-containing, and (3) α-tertiary primary amines, each of which previously required a different catalyst to achieve optimal results.

Metal free amination of congested and functionalized alkyl bromides at room temperature

Herein, we report a highly facile and unprecedented approach to synthesize congested N-(hetero)aryl amines en route to α-amino acid amides using α-bromoamides as alkylating agents under mild reaction conditions (room temperature). The involvement of aza-oxyallyl cations as alkylating agents is the hallmark of this reaction. The method was readily adapted for the rapid synthesis of coveted 1,4-benzodiazepine-3,5-diones.

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature

Palladium allyl, cinnamyl, and indenyl complexes with the ylide-substituted phosphines Cy3 P+ -C- (R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3 P+ -C- (Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C-N coupling reactions. The complexes are highly active in the amination of 4-chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β-hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C-N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram-scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h-1 .

An Improved PIII/PV═O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps

Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C–N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1·[O]). The central observations include the following: (1) catalytic reduction of 1·[O] to P^III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by ³¹P NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by ¹⁵N NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T ≥ 60 °C), low catalyst loadings (≥2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C–N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C–N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1·[O] and formation of the target C–N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C–N coupling.

Dialkylterphenyl Phosphine-Based Palladium Precatalysts for Efficient Aryl Amination of N-Nucleophiles

A series of 2-aminobiphenyl palladacycles supported by dialkylterphenyl phosphines, PR₂ Ar' (R=Me, Et, iPr, Cyp (cyclopentyl), Ar'=Ar^Dipp2 , Ar^Xyl2f , Dipp (2,6-C6H3-(2,6-C6H3-(CHMe2)2)2), Xyl=xylyl) have been prepared and structurally characterized. Neutral palladacycles were obtained with less bulky terphenyl phosphines (i.e., Me and Et substituents) whereas the largest phosphines provided cationic palladacycles in which the phosphines adopted a bidentate hemilabile k¹ -P,η¹ -C_arene coordination mode. The influence of the ligand structure on the catalytic performance of these Pd precatalysts was evaluated in aryl amination reactions. Cationic complexes bearing the phosphines PiPr₂ Ar^Xyl2 and PCyp₂ Ar^Xyl2 were the most active of the series. These precatalysts have demonstrated a high versatility and efficiency in the coupling of a variety of nitrogen nucleophiles, including secondary amines, alkyl amines, anilines, and indoles, with electronically deactivated and ortho-substituted aryl chlorides at low catalyst loadings (0.25-0.75 mol % Pd) and without excess ligand.

Unraveling the High Activity of Ylide-Functionalized Phosphines in Palladium-Catalyzed Amination Reactions: A Comparative Study with CyJohnPhos and PtBu3

Comprehensive mechanistic insights into the activity of different catalysts based on different ligands are important for further ligand design and catalyst improvement. Herein, we report a combined computational and experimental study on the mechanism and catalytic activity of the ylide-substituted phosphine Cy₃P–C(Me)PCy₂ (keYPhos, L1) in C–N coupling reactions including a comparison with the established and often-applied phosphines ^CyJohnPhos (L2) and P(tBu)₃ (L3). Density functional theory (DFT) calculations together with the possible isolation of several intermediates within the catalytic cycle demonstrate that L1 readily forms low-coordinated palladium complexes [such as L1·Pd(dba)], which easily undergo oxidative addition and subsequent amine coordination as well as reductive elimination. Due to the possible opening and closing of the P–C–P angle in L1, the steric bulk can be adjusted to the metal environment so that L1 retains its conformation throughout the whole catalytic cycle, thus leading to fast catalysis at room temperature. Comparative studies of the three ligands with Pd₂dba₃ as a Pd source show that only L1 efficiently allows for the coupling of aryl chlorides at room temperature. DFT studies suggest that this is mainly due to the reluctance/inability of L2 and L3 to form the catalytically active species under these reaction conditions. In contrast, the YPhos ligand readily forms the prereactive complex and undergoes the first oxidative addition reaction. These observations are confirmed by kinetic studies, which indicate a short induction period for the formation of the catalytically active species of L1, followed by fast catalysis. This behavior of L1 is due to its unique electronic and steric properties, which support low activation barriers and fast catalyst generation.

High-Throughput Discovery and Evaluation of a General Catalytic Method for N-Arylation of Weakly Nucleophilic Sulfonamides

Through targeted high-throughput experimentation (HTE), we have identified the Pd/AdBippyPhos catalyst system as an effective and general method to construct densely functionalized N,N-diaryl sulfonamide motifs relevant to medicinal chemistry. AdBippyPhos is particularly effective for the installation of heteroaromatic groups. Computational steric parametrization of the investigated ligands reveals the potential importance of remote steric demand, where a large cone angle combined with an accessible Pd center is correlated to successful catalysts for C-N coupling reactions.

Biaryl Monophosphine Ligands in Palladium-Catalyzed C-N Coupling: An Updated User's Guide

Over the past three decades, Pd-catalyzed cross-coupling reactions have become a mainstay of organic synthesis. In particular, catalysts derived from biaryl monophosphines have shown wide utility in forming C-N bonds under mild reaction conditions. This work summarizes a variety of C-N cross-coupling reactions using biaryl monophosphines as supporting ligands, with the goal of directing synthetic chemists towards the ligands and conditions best suited for a particular coupling.

A Highly Active Ylide-Functionalized Phosphine for Palladium-Catalyzed Aminations of Aryl Chlorides

Ylide-functionalized phosphine ligands (YPhos) were rationally designed to fit the requirements of Buchwald-Hartwig aminations at room temperature. This ligand class combines a strong electron-donating ability comparable to NHC ligands with high steric demand similar to biaryl phosphines. The active Pd species are stabilized by agostic C-H⋅⋅⋅Pd rather than by Pd-arene interactions. The practical advantage of YPhos ligands arises from their easy and scalable synthesis from widely available, inexpensive starting materials. Benchmark studies showed that YPhos-Pd complexes are superior to the best-known phosphine ligands in room-temperature aminations of aryl chlorides. The utility of the catalysts was demonstrated by the synthesis of various arylamines in high yields within short reaction times.

Mono- and diylide-substituted phosphines (YPhos): impact of the ligand properties on the catalytic activity in gold(i)-catalysed hydroaminations

The monoylide-phosphines show a linear correlation between their donor strength and their activity in gold-catalysed hydroaminations, while steric congestions leads to lower activities of the diylide congeners despite their higher donor properties.

Nucleophilic Amination and Etherification of Aryl Alkyl Thioethers

A transition-metal-free protocol capable of synthesizing diarylated aniline derivatives is reported. This method could be further employed to prepare aryl alkyl ethers. A wide range of thioethers, anilines, as well as alcohols were tolerated thanks to the mild reaction conditions. The strength of our method was demonstrated by performing a gram-scale reaction (20 mmol) followed by conversion of the nitrile group into synthetically useful aldehyde, ketone, and carboxylic acid.

Total Synthesis of (-)-Nodulisporic Acids D, C, and B: Evolution of a Unified Synthetic Strategy

A unified synthetic strategy leading to the total synthesis of (-)-nodulisporic acids D, C, and B is described. Key synthetic transformations include a nickel-chromium-mediated cyclization, an aromatic ring functionalization employing a novel copper-promoted alkylation, a palladium-catalyzed cross-coupling cascade/indole ring construction, and a palladium-mediated regio- and diastereoselective allylic substitution/cyclization reaction, the latter to construct ring D.

Reactivity enhancement of a diphosphene by reversible N-heterocyclic carbene coordination

The reversible coordination of an N-heterocyclic carbene (NHC) enhances the reactivity of a diphosphene towards hydrolysis and transfer hydrogenation. The hydrolysis is catalytic in NHC.

Diphosphene Ter^MesP = PTer^Mes (1; Ter^Mes = 2,6-Mes₂C₆H₃; Mes = 2,4,6-Me₃C₆H₂) and NHC^Me₄2 (NHC^Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) exist in an equilibrium mixture with the NHC^Me₄-coordinated diphosphene 3. While uncoordinated 1 is inert to hydrolysis, the NHC adduct 3 readily undergoes hydrolysis to afford a phosphino-substituted phosphine oxide with the liberation of NHC^Me₄. On this basis, conditions suitable for the catalytic use of NHC^Me₄ were identified. Similarly, while the hydrogenation of free diphosphene 1 with H₃N·BH₃ is very slow, 3 reacts instantaneously with H₃N·BH₃ at room temperature to afford a dihydrodiphosphane.

Synthesis of 2-Aminopyridines via a Base-Promoted Cascade Reaction of N-Propargylic β-Enaminones with Formamides

N-Substituted formamides as nucleophiles react with in situ-generated 1,4-oxazepines from N-propargylic β-enaminones followed by spontaneous N-deformylation to deliver densely substituted 2-aminopyridines in good yields (31-88%). The formyl group is found to be a superior traceless activating group of free amines and would ultimately be removed in situ. This reaction proceeds smoothly at room temperature, in the presence of NaOH as sole additive, without protection from the atmosphere and generates H₂O and sodium formate as byproducts.

Breaking the Base Barrier: An Electron-Deficient Palladium Catalyst Enables the Use of a Common Soluble Base in C-N Coupling

Due to the low intrinsic acidity of amines, palladium-catalyzed C-N cross-coupling has been plagued continuously by the necessity to employ strong, inorganic, or insoluble bases. To surmount the many practical obstacles associated with these reagents, we utilized a commercially available dialkyl triarylmonophosphine-supported palladium catalyst that facilitates a broad range of C-N coupling reactions in the presence of weak, soluble bases. The mild and general reaction conditions show extraordinary tolerance for even highly base-sensitive functional groups. Additionally, insightful heteronuclear NMR studies using 15N-labeled amine complexes provide evidence for the key acidifying effect of the cationic palladium center.